This invention relates to structures for making nano-interconnected or nano-packaged circuits and methods of making same, and more particularly, to vertical electrical connection of circuits using conductive nanowires.
Nano-scale wires such as carbon nanotubes with a very small size scale, on the order of 1-100 nanometers in diameter and 0.1-100 xcexcm in length, have received considerable attention in recent years. See Liu et al, SCIENCE, Vol. 280, p. 1253 (1998); Ren et al., SCIENCE, Vol. 282, p. 1105 (1998); Li et al., SCIENCE, Vol. 274, p. 1701 (1996); Frank et al., SCIENCE, Vol. 280, p. 744 (1998); J. Tans et al., NATURE, Vol. 36, p. 474 (1997); Fan et al., SCIENCE, Vol. 283, p. 512 (1999); Collins et al., SCIENCE, Vol. 278, p. 100 (1997); Kong et al., NATURE, Vol. 395, p. 878 (1998); and Ebbesen et al., NATURE, Vol. 382, p. 54 (1996).
Carbon nanotubes exhibit unique atomic arrangements, nano-scale structures and interesting physical properties such as one-dimensional electrical behavior, quantum conductance, and ballistic transport characteristics. The ballistic transport in carbon nanotubes, as reported by Frank et al, allows the passage of huge electrical currents in electronic circuits, with the magnitude of current density comparable to or better than those in some superconductors. Carbon nanotubes are one of the smallest dimensioned nanowire materials with generally high aspect ratio and small diameter of xcx9c1 nm in the case of single-wall nanotubes and less than xcx9c50 nm in the case of multi-wall nanotubes. See Rinzler et al, APPLIED PHYSICS, Vol. A67, p. 29 (1998); Kiang et al, J. PHYSICAL CHEM., Vol. 98, p. 6612 (1994), and Kiang et al, PHYSICAL REVIEW LETTERS, Vol. 81, p. 1869 (1998).
High-quality single-walled carbon nanotubes are typically grown as randomly oriented, needle-like or spaghetti-like, tangled nanotubes by laser ablation or arc techniques (a chemical purification process is usually needed for arc-generated carbon nanotubes to remove non-nanotube materials such as graphitic or amorphous phase, catalyst metals, etc). Chemical vapor deposition (CVD) methods such as used by Ren et al., Fan et al., and Li et al tend to produce multiwall nanotubes attached to a substrate, often with a semi-aligned or an aligned, parallel growth perpendicular to the substrate. As described in these articles, catalytic decomposition of hydrocarbon-containing precursors such as ethylene, methane, or benzene produces carbon nanotubes when the reaction parameters such as temperature, time, precursor concentration, flow rate, are optimized. Nucleation layers such as a thin coating of Ni, Co, Fe, etc. are often intentionally added to the substrate surface to nucleate a multiplicity of isolated nanotubes. Carbon nanotubes can also be nucleated and grown on a substrate without using such a metal nucleating layer, e.g., by using a hydrocarbon-containing precursor mixed with a chemical component (such as ferrocene) which contains one or more of these catalytic metal atoms. During the chemical vapor decomposition, these metal atoms serve to nucleate the nanotubes on the substrate surface. See Cheng et al., CHEM. PHYSICS LETTERS, Vol. 289, p. 602 (1998).
The modern trend in electronic circuit design, interconnection and packaging is toward use of finer features, such that submicron feature sizes have been reached in recent years. To produce desired, ultra-high density electronic packaging, a small width of the circuit lines is important, as well as a three-dimensional, multi-layer configuration with vertically integrated circuit layers. However, nanowires grown with presently-available methods are not suitable for such purposes. The as-grown single-wall nanotubes (SWNT) such as commonly synthesized by laser ablation or arc method, have a spaghetti-like configuration and often are tangled with each other. The multi-wall nanotubes (MWNT), such as commonly made by chemical vapor deposition, are easier to prepare in an aligned and parallel configuration. However, these as-grown nanotubes such as reported by Ren et al. and Li, et al. differ in height or length. For reliable circuit interconnections without electrical shorts or opens, it is desirable to prepare nanowires having equal and specific predetermined lengths. Further, it would be advantageous to provide the nanowires as free-standing wires so that they may be manipulated, e.g., for transfer, placement and bonding for circuit interconnections at ambient or relatively low temperatures, e.g., below 300xc2x0 C. Selective CVD growth of nanowires such as carbon nanotubes directly on desired circuit pads may be possible using selective area patterning of a catalyst layer; however, often it is undesirable to expose the delicate semiconductor circuits and components to the high temperatures (e.g., 600-1000xc2x0 C.) and chemical environments involved with CVD deposition of nanotubes. The invention discloses substantially equal length nanowires that may be fabricated as free-standing units suitable for convenient vertical interconnections and vertically interconnect circuit devices using such nanowires.
The invention comprises a vertically-interconnected circuit device having at least two circuit layers and a plurality of substantially equi-length nanowires disposed therebetween. The nanowires may comprise composites, e.g., having a heterojunction present along the length thereof, to enable a number of device applications. Also disclosed is a method for making the circuit device comprising growing a plurality of nanowires on a removable substrate, equalizing the length of the nanowires (e.g., so that each one of the plurality of nanowires is substantially equal in length), transferring and bonding exposed ends of the plurality of nanowires to a first circuit layer; and removing the substrate. The nanowires attached to the first circuit layer can be further bonded to a second circuit layer to provide the vertically-interconnected circuit device.